[JFFS2] Fix return value check for mtd->point() in check_node_data()

If we ask it to map 'len' bytes of the device, don't compare against 
some other number and whine that it's different. That's a little silly.

Signed-off-by: Alexey Korolev <akorolev@infradead.org>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>